1.5" overall. Suitable for mounting mulitple glass tritium vials (tritium vials are not included). Each bore can hold (1) 3mm x 23mm vials or (2) 3mm x 1mm vials. 0.472" outside diameter. Comes with (5) rubber plugs for vial containment, (2) O-rings, and (2) split rings for attachment. Bulk packed.